TempStr = input("请输入带有符号的温度值: ")

while TempStr[-1] not in ['N','n']:   #输入N，n退出计算
    if TempStr[-1] in ['F','f']:
        C = (eval(TempStr[0:-1]) - 32)/1.8
        print("转换后的温度是{:.2f}C".format(C))
    elif TempStr[-1] in ['C','c']:
        F = 1.8*eval(TempStr[0:-1]) + 32
        print("转换后的温度是{:.2f}F".format(F))
    else:
        print("输入格式错误")
    TempStr = input("请输入带有符号的温度值: ")